Unless otherwise indicated, the foregoing is not admitted to be prior art to the claims recited herein and should not be construed as such.
Knowing the amount of current being delivered to a load can be useful in a wide variety of applications. For example, in low-power electronic devices (e.g., smart phone, computer tablets, and other consumer electronics) the supply current can be monitored to understand the system's impact on battery life. The load current also can be used to make safety-critical decisions in over-current protection circuits. Generally, a current sensor is a circuit that can detect a current (e.g., current through a load) and produce an output current that is representative of the detected current. In some circuit applications, the output current can be converted to an easily measured output voltage that is proportional to the detected current.
In typical current sensing circuit designs, it is important to be able to produce a sense current that accurately represents (replicates) the current flowing (the current being sensed) through the pass device that supplies current to the load. Analysis of accuracy limitations of producing a sense current of a current flowing through a pass device has shown that the replica device voltage drop across the channel must match the voltage drop across the channel of the pass device very accurately. Typically, an active high gain feedback loop is used, which employs one or more amplifiers. The offset in each amplifier should be reduced to very low values in order to produce an accurate sense current.
A technique called “auto-zeroing” can automatically drive the DC offset of an amplifier to zero. Auto zeroing uses a switched capacitor technique. The conventional switched capacitor auto zero technique is one that prevents the amplifier from being used during part of a repeating cycle during which a capacitor samples its offset. FIG. 1, for example, shows a basic switched capacitor auto zero design. At time φ1, switches S1 and S2 are closed and switch S3 is open, allowing the capacitor C to sense and store the DC offset Vos of amplifier A. At time φ2, switches S1 and S2 are open and switch S3 is closed, allowing the amplifier A to operate on signal x(t), using the voltage stored in capacitor C to cancel the DC offset of amplifier A. However, during time φ1, when the DC offset is being sensed by capacitor C, the amplifier A cannot be used to process signal x(t). This intermittency can degrade the performance of a circuit that uses amplifier A. Alternatives that do not exhibit this drawback use two opamps, each with two inputs. Such designs are therefore large, requiring more die area and consuming more power.